‘Fewer’ is used with countable nouns and ‘less’ is used withuncountable nouns. Hence, in the given sentence, ‘less’ should be used in the context of ‘rainfall’ and ‘fewer’ should be used in the context of ‘accidents’. Therefore, ‘Less rainfall means fewer traffic accidents’ is the correct replacement of ‘Fewer rainfallmeans less traffic accidents’. Thus, option (a) is correct.